This invention relates to glucose fuel cells for use in the body. In particular, this invention relates to improvements in the volumetric power densities of glucose fuel cells.
In a glucose-based biofuel cell, glucose is oxidized at the anode, while oxygen is reduced to water at the cathode. The nature of the catalyst residing at the anode influences the extent of glucose oxidation and the associated oxidation products.
Three major design paradigms for glucose-based fuel cells exist, with numerous design examples described in patent and scientific literature, differing principally in the materials used to catalyze electrode reactions. In a first paradigm, the catalysts are isolated enzymes that are fixed to electrode substrates. In a second paradigm, oxidation is performed by exoelectrogenic bacteria, in biofilms, that colonize a fuel cell anode. In a third paradigm, the catalysts are abiotic, solid-state materials.
Enzyme-based glucose fuel cells have high catalytic efficiency which, together with their small size, results in high volumetric power density, yielding up to 4.3 μW of total power in systems with footprints on the order of 1 mm2 and volumes less than 10−2 mm3. Such fuel cells are often constructed as tethered-enzyme systems, in which oxidation and reduction of fuel cell substrates are catalyzed ex vivo by enzymes molecularly wired to threads of conductive material. Enzyme-based glucose fuel cells described in the recent literature have typically generated on the order of 100 μW·cm−2. Fuel cells of this kind may be capable of continuous operation for up to several weeks. However, their lifetimes are often limited by the tendency of the enzymes to degrade and ultimately degenerate with time. This characteristic of enzymes is one of the main reasons why the body constantly makes and degrades enzymes, such that the enzymes never lose their efficacy.
Using living microorganisms, such as exoelectrogenic bacteria, to catalyze the anodic reaction results in complete oxidation of glucose, liberating twenty-four electrons per molecule of glucose consumed. Microbial fuel cells are therefore very catalytically efficient and can produce more than 1900 μW·cm−2. In contrast with enzymatic systems, which have shorter lifetimes and are limited by the degradation of tethered enzymes ex vivo, microbial fuel cells are inherently self-regenerating as microbial fuel cells use a fraction of the input biomass to power and supply molecular substrates for maintenance functions such as resynthesis of degraded enzymes. Microbial glucose fuel cells described in the recent literature have typically generated on the order of 1000 μW·cm−2. However, the prospect of implanting even non-pathogenic bacteria raises concerns of safety and biocompatibillty. Thus, microbial fuel cells of the present generation are not yet suitable for biologically implanted applications.
Solid-state anode catalysts are capable of oxidizing glucose to gluconic acid, liberating one pair of electrons, and yielding further oxidation products with reduced probability. As a result, solid-state catalysts represent the least catalytically efficient of the three design paradigms identified above. While glucose fuel cells based on solid-state catalysts typically only generate 1 to 10 μW·cm−2, they have proven reliable as implantable power sources in animals for several months due to their use of biocompatible materials. FIG. 1A illustrates biocompatible glucose fuel cells of various cross-sectional areas fabricated on a rigid silicon wafer substrate. As shown in FIG. 1B, one of the glucose fuel cells fabricated on the wafer generated an open circuit voltage of 192 mV, with steady state power of 3.8 μW·cm−2 and transient peak power levels in excess of 180 μW·cm−2 at 1.5 to 1.85 mA·cm−2. However, these power levels are still relatively small.
Hence, a need exists for biocompatible glucose fuel cells having improved volumetric power density for use in the human body for relatively long durations of time.